A green garlic (Allium sativum L.) based intercropping system reduces the strain of continuous monocropping in cucumber (Cucumis sativus L.) by adjusting the micro-ecological environment of soil

Abstract

The continuous cropping obstacle of cucumber (Cucumis sativus L.) under facility cultivation is more prevalent in China. This is associated with an imbalance in soil microbial and ecological environment in long-term monocultures. It was postulated that intercropping with green garlic would relieve the continuous cropping obstacle of cucumber by altering the soil micro-ecology status. A pot-based experiment was conducted to investigate the green garlic-cucumber intercropping and cucumber monocropping systems. The results showed that the cucumber shoot biomass was improved by intercropping with green garlic. However, the population of soil bacteria and actinomycetes increased, while the fungal population decreased. The fatty acid methyl ester (FAME) profiles indicated that soil contained more fungal FAME biomarkers (18:1ω9c, 18:2ω6, 9) and higher fungal:bacterial ratio in the monoculture system, whereas clustering of more bacterial FAME biomarkers (cy17:0, cy19:0, 16:1ω7c10, Me16:0, 10Me17:0, 10Me18:0) was observed under intercropping conditions. Moreover, significantly (P < 0.05) higher soil invertase and alkaline phosphatase activities, organic matter, and available N, P and K contents were observed under intercropping systems. These were high in both bulk and rhizosphere soils in the intercropping system when compared to monocropping system. These findings suggest that intercropping with green garlic can alleviate continuous cropping obstacle of cucumber by improving the diverse composition of soil microbial community, enzyme activities, and nutrient availability.

Keywords: Continuous cropping obstacle; Cucumber; Green garlic-based cropping systems; Soil micro-ecological environment.

Figure 1. Effect of intercropping with green garlic on cucumber shoot (A) and root (B) biomass after interplanting on days 15, 30 and 45.

MC-cucumber monoculture; IC-cucumber intercropping with green garlic. Data are means ± standard error of n = 3. For each sample date, columns with same letters are not significantly different at the 5% level.

Figure 2. Effect of intercropping with green garlic on the cucumber chlorophyll a (A), chlorophyll b (B) and carotenoids (C) after interplanting on days 15, 30 and 45.

MC-cucumber monoculture; IC-cucumber intercropping with green garlic. Data are means ± standard error of n = 3. For each sample date, columns with same letters are not significantly different at the 5% level.

Figure 3. Effect of intercropping with green garlic on the activities of soil catalase (A–B), invertase (C–D), urease (E–F) and alkaline phosphatase (G–H) in the bulk and rhizosphere soils of cucumber.

Samples were collected after interplanting on days 15, 30 and 45. CB-bulk soil from cucumber monocropping; GB-bulk soil from green garlic intercropping with cucumber; CR-rhizosphere soil from cucumber monocropping; and GR-rhizosphere soil from green garlic intercropping with cucumber. Data are means ± standard error of n = 3. For each sample date, lines with same letters are not significantly different at the 5% level.

Figure 4. Effect of intercropping with green garlic on the number of bacteria (A–B), fungi (C–D) and actinomycetes (E–F) in the bulk and rhizosphere soils of cucumber.

Samples were collected after interplanting on days 15, 30 and 45. CB-bulk soil from cucumber monocropping; GB-bulk soil from green garlic intercropping with cucumber; CR-rhizosphere soil from cucumber monocropping; and GR-rhizosphere soil from green garlic intercropping with cucumber. Data are means ± standard error of n = 3. For each sample date, lines with same letters are not significantly different at the 5% level.

Figure 5. Total fatty acid methyl esters (FAMEs), (moles%) in the bulk (A) and rhizosphere (B) soils of cucumber under intercropping and monocropping systems on different sampling dates.

Samples were collected after interplanting on days 15, 30 and 45. CB-bulk soil from cucumber monocropping; GB-bulk soil from green garlic intercropping with cucumber; CR-rhizosphere soil from cucumber monocropping; and GR-rhizosphere soil from green garlic intercropping with cucumber. Data are means ± standard error of n = 3. For each sample date, lines with same letters are not significantly different at the 5% level.

Figure 6. Box plots of relative abundance of indicator FAMEs in the rhizosphere and bulk soils of cucumber under intercropping and monocropping systems.

Each group is composed of one or more FAMEs and is associated with particular taxon (see Methods). The horizontal line in the box was the median, and the upper and lower “hinges” are the first and third quartiles, respectively. The upper and lower “whiskers” extend to the highest or the lowest value within 1.5 times the inter-quartile range (the distance between the first and third quartiles). A, B, C, D, E, F, G, H and I represent indicator Gram-positive bacteria, saprophytic fungi, total bacterial sums, Gram-negative bacteria, mycorrhizal fungi, fungal/bacterial ratio, actinomycetes, protozoa and Gram^+/ Gram-ratio. CB-bulk soil from cucumber monocropping; GB-bulk soil from green garlic intercropping with cucumber; CR-rhizosphere soil from cucumber monocropping; and GR-rhizosphere soil from green garlic intercropping with cucumber. Data are means ± standard error of n = 9 (three sample dates of three replications for every sample date). For each sample date, boxes with same letters are not significantly different at the 5% level.

Figure 7. Microbial community structure according to FAMEs biomarkers (G⁺, G⁻, actinomycetes, protozoa, AMF, total bacteria, total fungi, fungal: bacterial and G⁺: G⁻) comparing different treatments during each sampling date (A, B and C) and comparing different co-existence times for monocropping and intercropping systems (D and E) .

The bacterial biomarkers identified included six Gram positive (G⁺) bacteria (i14:0, i15:0, a15:0, i16:0, i17:0, a17:0); three Gram negative (G⁻) bacteria (cy17:0, cy19:0, 16:1ω 7c) and three actinomycetes (10Me16:0, 10Me17:0,10Me18:0). The fungal markers included two saprophytic biomarkers (18:1ω 9c, 18:2ω 6, 9) and an arbuscular mycorrhizal fungi (AMF) associated biomarker (16:1 ω 5c). The total fungal sum was calculated based on the summation of 18:1ω 9c and 18:2ω 6, 9. The protozoan marker was used as 20:4ω 6c. CB-bulk soil from cucumber monocropping; GB-bulk soil from green garlic intercropping with cucumber; CR-rhizosphere soil from cucumber monocropping; GR-rhizosphere soil from green garlic intercropping with cucumber; and first, second and third sampling after interplanting on days 15, 30 and 45. Data are means ± standard error of n = 3.